The present invention relates to the use of isophoronediamine isomer mixtures as a reactant in polyaddition resins selected from the epoxy resins and polyurethane/polyurea resins.
Isophoronediamine (IPDA) is 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, which in the commercially available product is a mixture of isomers. For the isomer present in about 75% of the mixture, a chair conformation with a cis arrangement of the equatorial amino group on the C.sub.1 atom and of the equatorial aminomethyl group on the C.sub.3 atom has been established; the isomer present in about 25% of the mixture is the trans isomer with equatorial amino and axial methylamino group (Die Angewandte Makromolekulare Chemie 153 (1987) 1-13). Both the commercially available isophoronediamine and the isophorone diisocyanate (IPDI) obtainable therefrom with unchanged isomer ratio are used in the epoxy resin sector and the polyurethane resin art.
Epoxy resins are prepolymers that contain two or more epoxide groups per molecule; the reaction of these resins with a series of curing agents leads to cross-linked polymers. Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition (1987), pages 547-563, provides a survey of the resins and curing agents as well as their use and the properties of the cured resins. According to the properties required for particular fields of application, epoxy resin formulations contain, apart from resin containing one or more epoxide groups and one or more curing agents, solvents, reactive diluents, plasticizers, fillers and inductors.
It is known for the curing of epoxy resins (for example those based on bisphenol A diglycidyl ether) to use, apart from numerous other aminic curing agents, also cyclo-aliphatic diamines, including commercially available isophoronediamine (IPDA). Reference is made for example to DE-OS 31 37 898 and GB 1,515,473 which are directed to epoxy resin compositions for the embedding of electrical components or for coating purposes, wherein commercially available isophoronediamine is also used as a curing component.
To obtain optimal properties of cured epoxy resins, it is usually important to achieve as high a degree of cure as possible. To reach this target, in the use of commercially available isophoronediamine according to Paintindia, May 1987, pages 25-28, two kinds of inductors are suitably used: namely those for influencing the processing time (pot life) and those for influencing the cross-linking density.
After stirring together an epoxy resin with an amine curing agent, epoxy resin systems are reactive and have a limited processing time. This is often a disadvantage if the amount charged cannot be processed in the time available. There was therefore a need for a cycloaliphatic amine curing agent that combines the advantages of the commercially available isophoronediamine having the isomer composition previously described at the start with a prolonged processing time. In addition to the extension of the pot life with maintenance of a good curing and stability of the epoxy resins, there was also an interest in lowering as far as possible the maximum temperature during curing since this is accompanied by a reduction of the shrinkage.
It is further known to react diisocyanates or prepolymers containing isocyanate groups, which in addition to urethane groups can also have urea, biuret, isocyanurate, carbonamide or carbodiimide structural elements, in a polyaddition reaction with diamines, including also commercially available isophoronediamine (Ullmann's Enzyklopadie der Technischen Chemie, 4. Auflage, Band 19, 301-317, in particular pages 302, 306, 308-310). As a result of the use of the diamines as the reactant in the systems mentioned, urea groups are introduced into the macromolecule. This is a so-called amine-chain extension wherein polyurethane ureas are formed.
As a result of the high reactivity of amino groups towards NCO groups, sterically hindered amines or diamines with electron-attracting substituents are frequently preferred for chain extension. A requirement therefore exists for extending the range of diamines with reduced reactivity as reactants for the synthesis of polyurethanes/-ureas by polyaddition in order to remove the problems caused by the high reactivity of known amines, such as inhomogeneities due to a too short pot life and a reduced selectivity in the reaction.